The present invention relates generally to multi-phase resonant power converters. More particularly, the present invention relates to a multi-phase resonant power converter having circuitry and associated control methods to continuously adjust a resonant inductance and effectively balance the various phase currents.
Resonant converters have recently become progressively popular in power conversion, as zero volt switching and/or zero current switching can be achieved at the primary and/or secondary sides of the converter. Accordingly, high efficiency can be obtained due to lower switching power consumption.
To deliver more power, multi-phase resonant converters have been developed. Besides higher efficiency with phase management during light load conditions, another key benefit with multi-phase topologies is output ripple cancellation. Using the example of a single phase LLC converter, the ripple current may be up to 48.3% of the output current. However, the ripple current is reduced to 9.8% when two-phase interleaved LLC converters are utilized. Furthermore, three-phase interleaved LLC converters can achieve 4.2% ripple current. By calculation, the ripple current may be as low as 1% when six-phase interleaved LLC converters are adopted, meaning that no output capacitive filter would theoretically be required.
Unfortunately, the phase current may not be perfectly balanced because of resonant parameter sensitivity. Component tolerances, parameter variations with temperature, and loading and circuit parasitics all contribute to current imbalances among the various phases. In the worst case, the unit may fail due to overheating of the most heavily loaded phase. Phase current imbalance is therefore a very common issue in multi-phase resonant converters.
Referring to FIG. 7, an exemplary interleaved LLC converter 1 is represented as is known in the art. As is known in the art, the resonant frequency is given in accordance with the below Formula 1, where Lr stands for the inductance of the resonant inductor and Cr is the capacitance of the resonant capacitor (Cr is the parallel connection of Cr1 and Cr2, Cr3 and Cr4):
      Fr    =          1              2        ⁢                                  ⁢        π        ⁢                  LrCr                      ;Lr=Lr1=Lr2;Cr=Cr1+Cr2;Cr1=Cr2=Cr3=Cr4To achieve higher efficiency, the switching frequency should be set as close as possible to the resonant frequency.
The resonant inductor is the decisive component in an efficiency-optimized LLC circuit, which is reflected in that accurate capacitors (for example +/−5%) are readily available, but it is far more difficult to build accurate inductors without any trimming of the inductor (for example <+/−10%). Because not only the resonance frequency needs to be precise, but also the characteristic impedance of the resonance network (sqrt(Lr/Cr)), both components should be available with a low tolerance to assure a well balanced current in both phases. Therefore there is a need to have an adjustable resonance inductor.
It would therefore be desirable to provide a system and method which focuses on effectively reducing component sensitivity in the resonant inductance to achieve an optimal current balance in the multi-phase LLC converter.